This invention relates to a device for measuring convergence of three primary color electron beams of red, green and blue of a color cathode ray tube.
The color cathode ray tube (referred to as CRT hereinafter) displays a color image by directing each of the red, green and blue electron beams, controlled by video signal, correspondingly to the red, green and blue phosphor dots on the inner surface of the faceplate. The faceplate is composed of numerous red, green and blue phosphor dots deposited in a mosaic pattern.
In adjusting a color CRT, its convergence is measured and adjusted so that a properly colored display is obtained by assuring that the red, green and blue electron beams pass through any mesh of the shadow mask and accurately strike respective color phosphor dots. It is well known that convergence measurement devices are conventionally used to adjust convergence of the color CRT. Japanese Unexamined Patent Publication No. 59-747880 discloses a device in which a white composite test pattern for measurement is presented on the color CRT for convergence measurement; the test pattern is separated into red, green and red patterns using color filters; each separated pattern is then measured using an industrial television camera of which output is then used to calculate a luminous center of the test pattern of each color; and calculated relative positional difference is considered as a misconvergence to be compensated for.
The convergence measurement using a measurement device mentioned above requires a longer measuring time because the red, green, blue color test patterns are required to be separately picked up prior to the convergence calculation.
The longer measuring time causes the measurement device to operate under undesirable measurement condition changes such as supply voltage variations and luminance changes of the CRT due to generation of external noise or due to disagreemnet of the scan timing of the CRT and the scan timing of the image pickup device while the red, green, blue color patterns are individually picked up. Consequently, measurement accuracy may be degraded.
Also, the conventional device needs a fixture to mechanically keep both the color CRT and the industrial television camera securely in place during color filter changing operations. The fixture causes the size of the device to be larger.
Minolta Camera Kabushiki Kaisha, an asignee of the present application, has filed Japanese Patent Application No. 62-259088 which discloses a convergence measurement device using a color image pickup device to solve the above-mentioned problems. This convergence measurement device enables red, green and blue test patterns to be measured together at a time while, providing one shorter measuring time. The device also assures easy measurement operation of simply placing the color filter built-in image pickup device against the viewing screen of the CRT. The test pattern on the color CRT are produced by red, green, blue phosphor dots which are periodically spaced and mosaically arranged on the viewing screen of the CRT. In a color image pickup device provided with a single filter plate having mosaically-arranged three color filters, also, a great number of pixels are periodically spaced or distributed. It should be noted that a single color phosphor dot is picked up by a plurality of three color pixels. Accordingly, the measurement in which red, green, and blue test patterns are separately processed involves errors due to positional relation between the arrangement of the color phosphor dots on the color CRT and the arrangement of color pixels in the color image pickup device. When the color image pickup device is manually handled for quick measurement of convergence, different measurements are liable to generate due to the fact that the color image pickup device is unavoidably placed on different positions on the viewing screen.
Reasons why such an error takes place will be seen from the following description. FIG. 27 shows a cross hatch pattern displayed on the color CRT. To measure convergence, a horizontal misconvergence is obtained by a vertical line of the cross hatch pattern and a vertical misconvergence is obtained by a horizontal line of the cross hatch pattern. An area indicated at A in FIG. 27 is expanded in FIG. 28 to show in detail. Small circles represent regularly arranged phosphor dots. Phosphor dots given letters R, G, and B represent red phosphor dots, green phosphor dots, and blue phosphor dots respectively. A first area where blue phosphor dots are glowing is enclosed by two parallel alternate long and short dash lines. A second area where red phosphor dots are glowing is enclosed by two parallel alternate long and two short dash lines. A third area where green phosphor dots are glowing is enclosed by two parallel dashed lines. Pt is a vertical pitch between the phosphor dots of one color. FIG. 28 shows a part of the viewing screen of an unadjusted color CRT in which red, green, and blue luminance lines are still misconverged. A horizontal misconvergence is obtained by calculating respective luminous centers of gravity of misconverged red, green, and blue glowing lines to provide respective positional data of the three color test patterns, and by calculating a difference between the positional data. In the same manner as above, a vertical misconvergence is obtained in an area indicated at B FIG. 27.
FIG. 29 shows an arrangement of pixels of a color image pickup device. It is noted that a CCD (Charge Coupled Device) has a color filter alternately striped with red, green, and blue parts, and that pixels are arranged vertically as well as horizontally in respective uniform pitches. The pixels given letters R, G, and B represent red pixels, green pixels, and blue pixels respectively. Pt' is a horizontal pitch between the pixels of one color. Since Pt' is greatly smaller than color phosphor dot pitch Pt shown in FIG. 28, the plurality of red, green, and blue pixels receive light from the glowing phosphor dot when the color image pickup device is picking up the test pattern.
It is now assumed that a horizontal luminous center of gravity of the green phosphor dots is being calculated. The plurality of red, green, and blue pixels receive light from one green phosphor dot. However, the green pixels receive the light most strongly. Consequently, the image pickup device gives a color image of green stripes enclosed by a circle as shown in FIG. 30A and FIG. 30B. Each circle represents a glowing green phosphor dot and vertical hatched stripes inside the circle are a resultant image by the green pixels.
FIG. 30A and FIG. 30B show that the luminous center of gravity of the green phosphor dot image does not agree with the luminous center of the green phosphor dot itself. Two different phosphor dot images on two different columns give different results in the luminous center of gravity, although the green phosphor dot images on the same column agree to each other in luminous center of gravity. This is because green stripes (hatched) on the right hand side column and those on the left hand side column are out of phase as shown in FIG. 30A.
Deviation thus results as shown in FIG. 30A. A line Ms represents the luminous center of gravity of the glowing green phosphor dots themselves and a line Mm represents the luminous center of gravity of the glowing green phosphor dot image on the image pickup device.
If a deviation occurs in the horizontal positional relation between the pixels of the image pickup device and the color phosphor dots of the CRT under test, the above mentioned Mm varies. Accordingly, it could be seen that deviation .delta. between Mm and Ms changes each time the color image pickup device is moved. FIG. 30B shows that a horizontal movement of the image pickup device relative to the phosphor dots causes the deviation .delta. to change with respect to that of FIG. 29A.
When convergence is measured from the luminous center of color phosphor dot line of each color, a slight movement of the color image pickup device relative to the color phosphor dots at each measurement causes convergence data to vary, degrading measurement accuracy. This is mainly due to relative changes in positional relation between the arrangement of phosphor dots on the viewing screen of the CRT and the arrangement of the pixels in the color image pickup device. Experimentally, it is found that a relation of a phosphor dot pitch of 310 .mu.m (Pt) and a pixel pitch of 30 .mu.m (Pt') causes horizontal convergence data to vary in the term of 30 .mu.m or more.
Also, changed positioning of the image pickup device varies the position of calculation area for luminous center of gravity, consequently causing an increased measurement fluctuation.